Cr  K edge x-ray absorption study of Cr dopants in Mg2SiO4 and Ca2GeO4

X-ray absorption above the Cr K edge has been used to study two Cr-doped olivine-structure materials. For Mg2SiO4, the extended fine structure shows that Cr resides in octahedral and tetrahedral sites with Cr–O distances of 1.98±0.03 and 1.68±0.03 Å, while the near edge structure indicates that Cr is in 3+ and 4+ oxidation states. For Ca2GeO4, site-size constraints limit Cr to the tetrahedral site with a Cr–O distance of 1.745±0.02 Å, and this Cr is only in the 4+ oxidation state. This study illustrates the utility of x-ray absorption for probing and relating the oxidation states and sites of constituent elements. In this specific case of Cr in olivine hosts, such understanding is critical to the development of these materials as tunable infrared solid-state lasers: Cr in the 4+ state provides the desired emission centered near 1.3 μm.